TLDR, Would you consider an 'underlay' filter or perhaps an option on the 'overlay' filter that reverses the order of the input labels?
Consider the following shorthand syntax "[input][a] overlay [b] overlay [c] overlay [output]"
Which expands to "[input][a] overlay [unnamed1]; [b][unnamed1] overlay [unnamed2]; [c][unnamed2] overlay [output]"
Giving the final layer composition of "c,b,input,a"
If there was an underlay filter or equivalent the shorthand syntax "[input][a] overlay [b] underlay [c] underlay [output]"
Would expand to "[input][a] overlay [unnamed1]; [b][unnamed1] underlay [unnamed2]; [c][unnamed2] underlay [output]"
Giving the final layer composition of "input+a+b+c"
I am aware that I can provide longhand syntax to achieve the same result but I am programmatically apply a varying number of watermarks and would like to avoid a lot of temporary labels.
If you're open to the idea I'm willing to code it.